1. Field of the Invention
The present invention relates generally to a vacuum monitor for detecting abnormally high vacuum pressure within a vacuum interrupter, and more specifically to a vacuum monitor for deriving an alarm or indicating a caution when vacuum pressure within a vacuum interrupter is abnormally increasing.
2. Description of the Prior Art
Generally, a vacuum interrupter for use with an electric power circuit has a normal circuit interruption performance when the vacuum pressure within an evacuated envelope of the interrupter is kept below 10.sup.-4 Torr (Torricelli). However, the vacuum pressure sometimes increases causing deterioration of circuit interruption performance. The pressure increases and interruption deterioration occur because of, for instance, outgassing from materials used for the interrupter or slow leakage of air (air leaks through cracks caused by undue mechanical stresses or through welded or insufficiently brazed junction portions). In case vacuum pressure increases abnormally within the evacuated envelope, the small contact spacing is no longer able to sustain a high voltage applied to the contacts; arcs and flashovers occur; a white hot arc burns the contact surfaces and may melt the vacuum envelope and other parts of the vacuum interrupter.
Therefore, in an electric power circuit, it is very important to check or monitor pressure within the vacuum interrupter while a vacuum interrupter is in operation; namely, even while the contacts are kept closed or opened. In recent years, accordingly, various kinds of vacuum pressure monitoring devices for vacuum interrupters have been put into practical use. These vacuum pressure monitoring devices, however, have the following aspects and disadvantages in practical use: (1) There exists a vacuum pressure measuring device having an additional pair of electric discharge electrodes in the evacuated envelope; a high voltage is independently applied to the electric discharge electrodes by a separate high-voltage power supply to check the change in electric discharge phenomenon caused in accordance with Paschen's law (because vacuum pressure exerts an influence upon the phenomenon of electric discharge). In such a device, the structure of the vacuum interrupter is complicated and the manufacturing cost is relatively high, because an additional high-voltage power supply is necessary. (2) Another vacuum interrupter pressure measuring method involves disconnecting the interrupter from a power circuit and spacing movable and fixed contacts thereof so that an electric discharge is readily generated in accordance with Paschen's law; then a high voltage is applied to the spaced contacts from a separate high voltage power supply, to check the state of electric discharge. In such a method, the power circuit or the power supply must be disconnnected from the vacuum interrupter; thus, it is impossible to check the vacuum pressure while a high voltage is applied to the interrupter (i.e., under hot-line conditions) whereby the method is very time consuming and requires troublesome labor.